1. Technical Field
The present application relates to radio resource control including changing an amount of radio resource for use in radio communication between a base station and a mobile station.
2. Background Art
With the popularization of cellular phones, there is an increasing demand for voice communication and data communication. Seventy percent of the entire communication traffic is generated during communication indoors, for example, in users' houses, small-scale offices, and commercial facilities. To meet such an increasing demand, a small base station that can be installed indoors is currently under development. The coverage area of such a small base station is extremely smaller than that of a base station installed outdoors (hereinafter referred to as “macro base station”), so the coverage area of the small base station is called a femtocell, and the small base station is called a femtocell base station. The femtocell base station can accommodate high demand traffic as described above, and can be installed in locations where it is difficult for radio waves to penetrate, such as upper floors of buildings and underground malls. For this reason, the femtocell base station is now attracting attention as means for expanding the coverage area (communication area that satisfies a desired quality).
The use of the femtocell base station in radio communication standards for cellular phones, such as Wideband Code Division Multiple Access (W-CDMA) and Evolved Universal Terrestrial Radio Access (E-UTRA) of 3rd Generation Partnership Project (3GPP), and in radio communication standards for wireless metropolitan area network (wireless MAN), such as IEEE 802.16m, is now under review. In the W-CDMA standards, the femtocell base station is called Home NodeB (HNB). In the 3GPP E-UTRA standard, which is also called Long Term Evolution (LTE), the femtocell base station is called Home eNodeB (HeNB).
When the femtocell base station is used in a W-CDMA system, data transmission and reception using individual channels for performing transmission power control in an uplink and a downlink, and data transmission and reception using a shared channel in a downlink are carried out. Further, when the femtocell base station is used in an E-UTRA system, a radio frequency band is divided into a plurality of resource blocks (physical resource blocks (PRBs)). A scheduler included in a base station performs PRB allocation, and then data transmission and reception using the allocated PRBs is carried out. Furthermore, when the femtocell base station is used in an IEEE 802.16m system, OFDMA (Orthogonal Frequency Division Multiple Access) is adopted, and a radio frequency band is divided into subcarriers. Then, a scheduler included in a base station allocates the subcarriers, and data transmission and reception using the allocated subcarriers is carried out. A bundle of subcarriers corresponds to a resource block in E-UTRA.
The femtocell base station is connected to a backbone network (e.g. a core network of a carrier) through a femto gateway (femto GW). The femto GW is called Home NodeB Gateway in the W-CDMA standard and is also called Home eNodeB Gateway in the E-UTRA standard. When a mobile station is present in a femtocell, a mobile station registered in a femtocell base station can be connected to a network through the femtocell base station. On the other hand, a mobile station which is not registered in a femtocell base station cannot be connected to a network through the femtocell base station, or communication with the femtocell base station is limited compared to a registered mobile station. Hereinafter, a mobile station that is registered in advance in a femtocell base station is referred to as “registered mobile station”, and a mobile station that is not registered in advance in a femtocell base station is referred to as “non-registered mobile station”. Additionally, a mobile station that connects to and communicates with a femtocell base station is referred to as “femto mobile station”, and a mobile station that connects to and communicates with a macro base station is referred to as “macro mobile station”.
Base stations (for example, a macro base station, a micro base station, and a pico base station) in an existing mobile communication network transmit a reference signal (also referred to as “pilot signal”) to an area that is covered by the base stations. Mobile stations receive the reference signal, establish synchronization, and estimate a channel, for example, thereby transmitting and receiving data to and from the base stations. Accordingly, it is requisite that the mobile stations can receive the reference signal with a satisfactory quality in order to provide a satisfactory communication quality. This also holds true for the femtocell base station.
When a femtocell base station is installed, inter-cell interference (ICI) between a femtocell and existing macro base stations and other adjacent femtocell base stations occurs. In particular, when these base stations use the same frequency band, ICI becomes significant. For example, there is a possibility that the femtocell base station causes downlink interference in the mobile station that communicates with another femtocell base station in an adjacent room, which results in an adverse effect on the communication of the mobile station. As methods for avoiding ICI, a transmission power control, a radio resource control on a frequency axis (i.e. frequency resource control), and a radio resource control on a time axis (i.e. time resource control) are generally devised.
For example, International Patent Publication No. WO 2009/047972 (hereinafter referred to as “PTL 1”) discloses a transmission power control method as described below. First, a femtocell base station measures a reception quality of a reference signal received from a macro base station, and sets initial values of its own transmission power (transmission power of a reference signal and a maximum value of the transmission power) by adding a power offset to the measured value. Next, the femtocell base station receives, from a registered mobile station, a report including a measurement result as to a reception quality of a downlink signal (reference signal) received from the femtocell base station, and adjusts the transmission power of the femtocell base station so that the reception quality in the registered mobile station gets close to a target level. In other words, the femtocell base station disclosed in PTL 1 reduces its own transmission power when the reception quality of the downlink signal in the registered mobile station connected to the femtocell base station is higher than the target level, thereby reducing ICI to the adjacent femtocell base station.
3GPP contribution R1-103458, “Analysis on the eICIC schemes for the control channels in HetNet”, TSG RAN WG1 #61bis meeting, July, 2010 (hereinafter referred to as “NPL 1”) discloses examples of the frequency resource control and the time resource control. FIGS. 20A and 20B show an example of the frequency resource control. FIGS. 20A and 20B illustrate an example in which a PDSCH (Physical Downlink Shared Channel), a PDCCH (Physical Downlink Control Channel), and a reference signal are allocated within a 1 msec sub-frame of the E-UTRA when the same frequency band is used by two adjacent femtocell base stations. The PDSCH is a shared data channel for transmitting user data in a downlink. The PDCCH is a control channel for transmitting scheduling information for a downlink, such as a frequency arrangement, a modulation scheme, a data amount, and retransmission information. In the example shown in FIGS. 20A and 20B, a frequency band for downlink channels is divided into two frequency segments so as to prevent frequencies from overlapping each other between two adjacent femtocell base stations. Thus, these two femtocell base stations use different frequency segments within a single frequency band, thereby avoiding ICI.
International Patent Publication No. WO 2008/105091 (hereinafter referred to as “PTL 2”) discloses a technique in which two base stations that manage two adjacent cells perform switching between a first frequency allocation pattern and a second frequency allocation pattern according to the time. In the first frequency allocation pattern, these two cells use the same frequency (e.g. f1 and f2). In the second frequency allocation pattern, these two cells use different frequencies (e.g. one of the cells uses f1, and the other cell uses f2). Specifically, a base station transmits downlink signals while switching the frequency allocation pattern, and mobile stations measure a reception quality (radio quality) of downlink signals received from the base station and report the measured reception quality to the base station. The base station collects measurement results of the radio quality from a plurality of mobile stations belonging to its own cell, determines, for each terminal, which one of the frequency allocation patterns satisfies a desired radio quality, and allocates each terminal to one of the first and second frequency patterns. In general, a mobile station positioned near the center of the cell performs communication during the time of the first frequency allocation pattern in which more frequencies can be used, and a mobile station positioned at the cell boundary performs communication during the time of the second frequency pattern in which interference with an adjacent cell is less likely to occur.
Japanese Unexamined Patent Application Publication No. 2010-206794 (hereinafter referred to as “PTL 3”) discloses a technique in which a femtocell base station performs dynamic Inter-Cell Interference Coordination (ICIC).
Specifically, the femtocell base station acquires channel state information and interference information of downlink channels from a mobile station, and allocates, so as to optimize the performance of a network by using such information, time and frequency resources for transmitting data to the mobile station on the downlink channel. For example, when transmission powers of femtocell base stations are small, two adjacent femtocell base stations use a modulation scheme in which a distance between symbols is large and a desired SINR (Signal-to-Interference plus Noise power Ratio) is low (e.g. BPSK (Binary Phase Shift Keying)). To set a throughput to be constant, most radio resources on a time axis and a frequency axis are used, so that the radio resources used by the two adjacent femtocell base stations overlap each other. On the other hand, when transmission powers of femtocell base stations are large, two adjacent femtocell base stations use a modulation scheme in which the distance between symbols is small and the desired SINR is high (e.g. 16QAM (Quadrature Amplitude Modulation)). Since the required radio resources for the constant throughput can be reduced, thereby preventing the radio resources used by the two adjacent femtocell base stations from overlapping each other.
However, if the transmission power control or the frequency division, which are described above, are applied in a situation where femtocell base stations are installed at locations close to each other, unfairness in communication quality occurs between the femtocell base stations, and a decrease in frequency use efficiency occurs, for example. As an illustrative situation where femtocell base stations are installed at locations close to each other, there is a situation where the femtocell base stations are installed in adjacent houses (rooms) within an apartment house.
Problems will be described with reference to FIGS. 21A and 21B. Attention is focused on two adjacent users' houses 9A and 9B within an apartment house. Femtocell base stations 91A and 91B are installed in the user's houses 9A and 9B, respectively. The femtocell base stations 91A and 91B are communicating with registered mobile stations 92A and 92B, respectively. The mobile station 92B in the user's house 9B is a non-registered mobile station for the femtocell base station 91A in the user's house 9A. Similarly, the mobile station 92A in the user's house 9A is a non-registered mobile station for the femtocell base station 91B in the user's house 9B. Referring to FIG. 21A, the mobile stations 92A and 92B are positioned in the user's houses 9A and 9B, respectively. Referring to FIG. 21B, the mobile station 92A visits the user's house 9B.
When the base station 91A and 91B execute the transmission power control in accordance with the technique disclosed in PTL 1, the level of ICI occurring in the mobile stations 92A and 92B is low in the case of FIG. 21A, so that a satisfactory communication quality can be ensured. In the case of FIG. 21B, however, the mobile station 92A (non-registered mobile station for the femtocell base station 91B) which has visited the user's house 9B receives significant interference from the femtocell base station 91B. This may result in deterioration in the communication quality of the mobile station 92A, and may result in not being able to establish communication between the mobile station 92A and the femtocell base station 91A. Further, in the case of FIG. 21B, the communication quality of the femtocell base station 91B is maintained in the satisfactory state, but the communication quality of the femtocell base station 91A may deteriorate due to the influence of ICI. In short, when the technique disclosed in PTL 1 is employed, unfairness in communication quality occurs between the femtocell base stations 91A and 91B in the case of FIG. 21B.
On the other hand, when the base station 91A and 91B execute the frequency division in accordance with the technique disclosed in NPL 1, ICI received by the mobile station 92A, which has visited the user's house 9B, from the femtocell base station 91B can be alleviated in the case of FIG. 21B. This makes it possible to prevent the communication of the mobile station 91A from being disabled and to ensure the fairness in the communication quality between the femtocell base stations 91A and 91B. In the case of FIG. 21A, however, the frequency resources available for the mobile stations 92A and 92B are reduced due to the band division, with the result that the communication capacity (throughput) of each femtocell is reduced.
PTL 2 merely discloses that base stations constantly perform the operation for switching the frequency allocation pattern according to the time, and fails to disclose a technique for determining whether or not to perform the operation for switching the frequency allocation pattern, based on the communication quality. Accordingly, as with the technique disclosed in NPL1, in the technique disclosed in PTL 2, the use efficiency of the frequency resources decreases and the communication capacity (throughput) of each femtocell is reduced in the case of FIG. 21A. Further, when the operation for switching the frequency allocation pattern as disclosed in PTL 2 is carried out, the frequency allocation pattern using all available frequencies (e.g. the first frequency allocation pattern using f1 and f2) cannot be substantially used in the case of FIG. 21B, which results in a decrease in the use efficiency of the time resources and reduction in the communication capacity (throughput) of each femtocell.
When the dynamic ICIC disclosed in PTL 3 is constantly carried out, in the case of FIG. 21A, the femtocell base stations 91A and 91B reduce the transmission power and utilize overlapping radio resources, so that an improvement in the use efficiency of the frequency resources can be expected. In the case of FIG. 21B, the femtocell base stations 91A and 91B increase the transmission power and avoid the use of overlapping radio resources, so that suppression of ICI can be expected In the case of FIG. 21B, however, even if the transmission powers of the base station 91A and 91B are equally high, a propagation loss between the femtocell base station 91A and the mobile station 92A is considerably larger than a propagation loss between the femtocell base station 91B and the mobile station 92B. Accordingly, there is a possibility that the femtocell base station 91A and the mobile station 92A obtain a throughput relatively lower than that of the femtocell base station 91B and the mobile station 92B. In other words, when the dynamic ICIC disclosed in PTL 3 is employed, unfairness in the communication quality may occur between the femtocell base stations 91A and 91B in the case of FIG. 21B.
Though the situation in which a mobile station visits another user's house as shown in FIG. 21B is an assumed extreme case, a similar problem occurs in a situation where a mobile station receives strong ICI from a femtocell base station located in another user's house even when the mobile station does not visit another user's house.